Time-reversal has several powerful features that make it highly desirable for coherent radar imaging and tracking. These features include adaptive self-focusing of a beam on a target through distorting media, and high resolution imaging due to multipath-enabled extended virtual aperture. However, its implementation at high frequency over-the-horizon-radar (HF-OTHR), e.g. as described in J. F. Thomason, “Development of Over-the-Horizon Radar in the United States,” IEEE Radar 2003 Conference, Adelaide. Australia, pp 599-601 (Sep. 3-5, 2003) and J. M. Headrick, “HF Over-The-Horizon Radar,” Radar Handbook, 2d ed., Ch. 24. McGraw Hill Book (1990), is hampered by the required long RF cables which can raise various issues such as signal attenuation, phase instability due to thermal expansion, and difficulties in installation. If each element of HF-OTHR can be separate, independent, and modularized, its time-reversal implementation over a very large area can be greatly facilitated, significantly improving the performance.
Netted radars, such as WSN (wireless sensor network) are described in T. E. Derham, K. Woodbridge, H. Grifiths and C. J. Baker, “The design and development of an experimental netted radar system,” Radar Conference, Proceedings of the International, pp 293-98 (3-5 Sep. 2003) (“Derham 1”) and T. E. Derham. S. Doughty. K. Woodbridge and C. J. Baker, “Design and evaluation of a low-cost multistatic netted radar system,” Radar, Sonar & Navigation, IET, Vol. 1, Issue 5. pp. 362-68 (October 2007) (“Derham 2”), do not exhibit coherent integration of unwired remote radars both temporally and spatially.
Temporal coherence can be attained with current technology, e.g. as described in D. Trizna, and Lillian (Xialon) Xu, “Target Classification and Remote Sensing of Ocean Current Shear Using a Dual-Use Multifrequency HF Radar,” IEEE J. of Oceanic Eng. Vol. 31, No. 4, pp. 904-918 (October 2006) (“Trizna et al.”) using a GPS-disciplined Rubidium clock. However, spatial coherence with sub nsec (<0.2 nsec) synchronization among distributed sensors has proven difficult. The demonstrated coherent netted radar system to date (e.g. Deram 1 and Derham 2, supra) are “hard-wired” to meet the stringent requirements of coherence and synchronization.